1. Field of the Invention
This invention provides improved non-woven mats of randomly oriented, low melting, metal fibers, for use in metal/polymer composites, and particularly for shaped metal/polymer composites.
2. Description of the Related Art
A variety of composites containing both metal and polymeric materials are known for use in many varied applications. Composites may include metal in the form of continuous sheet, perforated sheets, mesh, woven screen or non-woven webs of randomly distributed fibers. Similarly, polymer structures, combined with the various forms of metal, may include films, sheets, perforated sheets, woven material or non-woven layers with random fiber distribution. One important use for a metal/polymer composite is as a shield for electromagnetic and radio frequency waves. The interference caused by such waves in electronic devices is commonly referred to as electromagnetic interference (EMI) or radio frequency interference (RFI), hereinafter jointly referred to as EMI. EMI shielding is often placed around an EMI source to prevent it from radiating EMI and interfering with surrounding devices. Also, the devices themselves may be provided with EMI shielding in an effort to shield the device from incoming electromagnetic radiation.
Effective EMI shielding requires the formation of a uniform conductive enclosure around the EMI-sensitive or EMI-emitting device. A shielding layer, associated with the conductive enclosure, may be in the form of a continuous layer or a discontinuous grid, such as a metal mesh. Any enclosure formation process that significantly increases the maximum void dimension in the shielding layer, sometimes called the "slot effect", could cause faulty EMI shielding performance of the shielding material.
Previous disclosures reveal ways of producing and shaping sheet material that has EMI shielding capability, typically using an electrically conducting layer, which are required in many applications.
For example U.S. Pat. No. 3,727,292, (Nicely), discloses a non-woven unitary metallic sheet which is fabricated by extruding a molten stream from a metallic melt into an atmosphere which reacts to form a stabilizing film about the periphery of the metal stream. The spun metal filaments are allowed to solidify, and then collected as a nonwoven fibrous mass. The mass of filaments is then compressed into a sheet-like form, and given strength by binding all or selected adjacent fibers together.
U.S. Pat. No. 4,689,098 (Gaughan) discloses an EMI shielding sheet comprising a layer of nonwoven reinforcing fibers which supports a layer of metal whiskers or fibers formed from a ductile metal or metal alloy. Metal layer formation disclosed in U.S. Pat. No. 3,272,292 (Nicely) applies to U.S. Pat. No. 4,689,089 which provides a stampable EMI shielding sheet. Another stampable EMI shielding construction appears in U.S. Pat. No. 4,678,699 (Kritchevsky et al). This patent notes that, "The shielding layer must be able to maintain its shielding effectiveness upon stamping." Such a statement reflects the fact that stamping processes tend to disrupt fibrous networks, breaking the fibers which, in the case of EMI shielding, results in poorer shielding effectiveness of the metal layers.
Stamping is one method for forming shaped EMI shielding structures. This forming technology was developed in the metal industry for forming thin metal objects. It involves rapid, almost instantaneous application of mechanical force to distort a sheet into a shaped object. Stampable plastic/metal composite sheets may require heating, to soften the plastic surrounding the metal shielding layer, prior to stamping. This reduces the modulus of the plastic, allowing it to flow while the metal shielding composite responds to the high pressure, shaping force of the stamping press. The speed of this process demands high levels of ductility for the metal and high plasticity for the remainder of the composite, to absorb the applied force without rupture. This method, applied to sheet molding compound (SMC), provides automotive body panels and business machine housings using reinforced material comprising a non-woven, glass-fiber reinforcing layer, and a mat containing conductive fibers for EMI shielding, held together with a resin such as polyester. The SMC is a flat sheet prior to forming in compression dies of high tonnage presses. Material properties limit the use of SMC to simple, relatively shallow shapes. Conditions used for sharp draws, e.g. multiple rib formation in the shaped panel, may cause ripping of the shielding layer and reduction of EMI shielding performance.
As a substitute for stamping, the use of thermoforming or injection molding may be considered. Thermoforming, as it relates to the present invention, comprises heating a sheet and forming it into a desired shape. The process includes heating a thermoplastic composite sheet above its softening point, then using either air pressure or vacuum to deflect the sheet towards the surface of a mold until the sheet adopts the shape of the mold surface. Upon cooling, the sheet sets in the required shape allowing removal from the mold.
European patent EP 529801, commonly assigned with the instant application, discloses EMI shielding, add-on sheets, comprising carrier material with a metal fiber mat at least partially embedded in the carrier material. The add-on sheets provide EMI shielding to selected parts of a thermoformed structure. Successful use of these add-on sheets requires that they possess or develop porosity when thermoformed in contact with the thermoformable substrate blank to which they were applied.
Depending on the melting point of the metal fibers in the EMI shielding layer, it is possible for individual fibers to melt and rupture under stress, such as during stretching and shaping, with resulting protrusions or "bumps" of metal at the point of separation. This can adversely affect EMI shielding efficiency due to increased size of voids in the shielding layer, after thermoforming. In addition, metal bumps may form as conductive projections from the surface of the shielding layers in electronic housings. Such projections cause potential electrical shorting problems if they contact circuit elements or microdevices in the restricted space usually associated with electronic packages. Also bumps may adversely affect injected resin flow when the shielding composite is an insert for injection molding.
Several alternative solutions have been attempted to improve the effectiveness of conductive fiber based EMI shielding. The formation of pressure welds or sintered bonds between the fibers improves electrical conductivity, but reduces overall flexibility and extensibility of the welded mat. Composite metal-fiber/polymer sheets containing such sintered metal mats cannot be thermoformed without breaking many of the fibers themselves, the bonds between the fibers or both, thus drastically reducing the shielding properties at the higher stretch ratios commonly required in thermoformed parts.
With the increasing use of advanced, EMI-sensitive electronics, a need exists for improved materials for shaping into EMI shielding housings that reliably protect electronic packages. Methods to shape EMI shielding structures rely upon the use of moldable composites with ability to retain shielding capabilities even when complex shapes demand localized elongation of 300-500%. This condition is possible using composite structures of the invention comprising a thermoplastic sheet of carrier material supporting a layer of randomly distributed, low melting metal fibers stabilized against fiber rupture and formation of bumps during forming by means of a coating (fiber-coating) of a thermoplastic polymer. Composite sheets of the invention provide improved EMI shielding performance by maintaining the integrity of molten metal strands during thermal shaping involving deep drawing of the composite sheet.